In many shallow water environments, the near-surface geology is highly complex and the near-surface geology contains significant horizontal inhomogeneities. The effects of near-surface complexity and horizontal inhomogeneities can be commonly observed in seismic data acquired in shallow water, marine environments near river outlets and in the Arctic. The near-surface complexity and lateral variability in these areas introduce significant multiples and local lateral velocity anomalies in the seismic data. This complexity and lateral variability degrade a seismic image. Energy contained in the multiples obscures deeper reflection energy. Local lateral velocity anomalies defocus imaged reflectors. To improve the seismic image, the multiple generators need to be identified and local velocity variations need to be characterized. For the seismic data to identify the origins of the multiple generators and sample the local velocity variations, it needs to contain zero-offset data and near zero-offset data (i.e. small-opening-angle data). Small-opening-angle data provide the best timing estimates and the least phase uncertainty for reflection data and for the associated multiple events. Better timing estimates and reduced phase uncertainty benefit inversion and demultiple algorithms in shallow water environments by constraining the solutions with pre-critical-angle reflection data. It will typically be sufficient if these small opening angle data are obtained only from a near-surface portion of the subsurface.
FIG. 1 shows a plan view of a typical hardware layout used for 3D marine acquisition. This hardware example uses two source arrays 11, which will typically but not necessarily be air guns, and eight seismic streamers 12 to create sixteen tracks of seismic data with each pass of the vessel over the survey area. Seismic sensor sets (not shown), typically hydrophones, are spaced along the length of each streamer. Each seismic streamer is connected to the vessel via an umbilical 13 that provides power to the streamer and telemetry signals to and from the seismic sensor sets. Because of the need to pull the streamer through the water, the umbilical is constructed of materials capable of many thousands of pounds of bollard pull. A consequence of the umbilical's strength is that it is very heavy and has a significant in-water weight which is typically tens of kilograms per meter. To support this heavy weight, a buoy 14—called a cable head buoy—is located at the head of each streamer. The motion of the streamer umbilical and cable head buoy is decoupled from the streamer by vibration isolation module(s) 15, which attenuate longitudinal traveling energy. Because of the towing arrangement and the need to attenuate longitudinal traveling energy, the distance between the source and the nearest receiver is typically 100 m to 150 m for conventional marine, seismic data acquisition. For seismic data acquisition in water depths that are large compared to the minimum source-receiver distance, e.g. greater than approximately 300 m, this minimum source to receiver separation does not preclude the acquisition of small-opening-angle data. In more shallow water depths, however, virtually no data with small opening angles will be acquired.
Other typical hardware features shown in FIG. 1 include the cable tail buoys 16 and the air gun umbilicals 17. Typical dimensions are 250 m from the navigation point 18 to the center of the air gun array, then 125 m to the cable head boys, then another 25 m to the center of the first group of sensors (not shown), with the streamers being separated by 50 m spacing.
This deficiency in shallow marine data has been recognized by the seismic industry for a long time. U.S. Pat. No. 3,744,021 to Todd describes a method for simultaneously acquiring deep and shallow reflection data along a common profile line. U.S. Pat. No. 8,467,264 to Keers describes the use of mini-streamers associated with the air gun sources to acquire near-zero offset data. U.S. Pat. No. 8,958,266, issued from Patent application publication 2010/0002539 by Kragh, and U.S. Pat. No. 8,964,502, issued from Patent application publication 2011/0063947 by Norris, describe methods of extracting zero-offset data from near-field hydrophones located within the air gun source array. All of these methods for obtaining zero-offset and near zero-offset seismic data define one or at most a few seismic profiles for each pass of the seismic vessel over the survey area. In the cases where a few seismic profiles are created, all of these seismic profiles define tracks that are directly beneath the air gun source arrays or tracks that are immediately adjacent to the path of the air gun source arrays. The information generated by these seismic profiles is useful for improving 2D seismic data but improvements to 3D data are limited by the significant spacing of the tracks in a crossline direction. Typically for 3D seismic acquisition, this crossline spacing can be from 400 m to 1000 m. In other words, when the tow vessel in FIG. 1 turns and comes back along the next survey line, the spacing between that sail line and the previous sail line will typically be 400 m to 1000 m.
To improve 3D data using the near-surface information, a high resolution, near-surface track of data needs to be created for each seismic streamer for each pass of the seismic vessel over the survey area. This need is spoken to by U.S. Pat. No. 6,556,510 to Ambs. Ambs's approach is to place lightweight, energy efficient acoustic energy sources throughout a seismic receiver array. This approach is applicable for the purposes of streamer location as described by Ambs, U.S. Pat. No. 6,229,760, and Austad, U.S. Pat. No. 6,839,302, but acoustic sources that are capable of being embedded within a streamer cable are inherently small, low power sources and operate at frequencies well above the typical seismic frequency band of 2 Hz to 150 Hz. Consequently, the signals this type of acoustic source can generate do not penetrate the earths subsurface or their depth of penetration is limited to the first few centimeters of the earth's subsurface. This limit on the depth of penetration does not provide the data needed to identify the origins of multiple generators or to characterize the local, subsurface velocity variations. What is needed is an acquisition method that penetrates into the first 300 m to 500 m of the earth's subsurface and provides a high resolution image of the subsurface along the track of every streamer in the seismic receiver spread.